U.S. Pat. No. 6,996,865 to Sabin describes a mattress structure in which a support assembly comprises a support plate having an array of mounting holes each receiving a respective telescoping spring assembly. In U.S. Pat. No. 6,996,865, various embodiments of the spring assembly are identified by the reference numerals 20, 120, 220, 320, and 420. The various spring assembly embodiments are telescoping spring assemblies in the sense that each has an inner member arranged to be axially slidable within an outer tubular member, and a mechanical spring is arranged within the inner and outer members to bias the members in an extension direction. The Sabin patent teaches that it is desirable to preload the spring to control firmness. In order to achieve preload, Sabin discloses the use of a spacer (e.g., spacers 28, 128, and 228) engaging an end of the spring and one of the tubular members, wherein the spacer has an axial length chosen to provide a desired preload. The spring assemblies mounted on the support plate may have different preload characteristics achieved by using spacers of different lengths to vary the firmness of the mattress in different support zones.
Efforts to commercialize the invention in U.S. Pat. No. 6,996,865 have been met with certain design challenges. A significant challenge has been the fact that the preload applied to the coil spring is transmitted to the inner and outer tubular members. Under this design, the inner and outer members are forced into a fully extended condition (absent external loading) and are prevented from separating by engagement of opposing shoulder surfaces on the external wall surface of the inner member and on the internal wall surface of the outer member. As a result, the frictional engagement between the shoulders of the inner and outer members causes the spring assembly to be “sticky” at times, and the spring assembly does not always react as intended when an external load is first applied. Moreover, after an external load is removed from a compressed spring assembly, the biasing force of the coil spring immediately pushes the outer member in the extension direction, and the ensuing engagement of opposing shoulder surfaces on the inner and outer members generates a popping sound that is disruptive to a person at rest.